Abstract Proton-coupled electron transfer (PCET) is a ubiquitous mechanism in biology, serving as the basis for mediating steps involving biosynthesis of both primary and secondary metabolites, radical generation and transport, and the activation of substrates at cofactors. The control of highly reactive radical intermediates is achieved via coupling proton and electron transfer processes. Management of radicals in biology is of particular relevance to human health, as enzymes operating by PCET are therapeutic targets with wide-ranging (RNR), which performs reversible long-range charge-transfer that spans 35 and two subunits (? and ?) upon applications including chemotherapy, anti-retroviral drugs and anti-inflammatory agents. The proposed research program seeks to define PCET at a detailed mechanistic level by focusing on ribonucleotide reductase every turnover. This process occurs via a pathway of redox-active amino acids, rendering RNR a paradigm for the study of PCET in biology. An interdisciplinary approach integrates a suite of experimental methods encompassing biochemistry, transient spectroscopy, synthesis and electrochemistry to target three specific aims. Owing to the sensitivity of the coupling between the proton and electron, we seek to define how conformational gating targets the PCET pathway. We will concentrate on tyrosine dyads and triads of the PCET pathway and introduce canonical and non-native point mutations to address the effects of driving force, electrostatic local environment and hydrogen bonding interactions involving these tyrosine clusters. In tandem with these biochemical inquiries, studies of cofacially aligned tyrosine model dyads will be investigated to direct link between allostery and radical transport. A second specific aim targets PCET across the ? | ? protein define how the energetics of radical generation are affected by stacking and hydrogen bonding. These studies will uncover how conformational gating controls RNR activity at an atomistic level and thus will establish a interface with the goal of identifying critical residues that mediate proton transfer attendant to radical transport. The subunit interface of RNR is a critical nexus of the PCET pathway and provides an access point for therapeutics designed to affect enzymatic function. The third specific aim seeks to understand the interplay between allosteric activation in ?2 and reduction of the Y122? cofactor, 35 away in ?2. To address this directly, we will embark on an investigation of the kinetics and thermodynamics of PCET through ? by employing new photo?2s. We will extend our studies to a RNR class featuring a (Mn Fe ) state to initiate forward PCET IV IV through the enzyme by photoinitiating the active state for radical generation. Together, the principles that emerge from addressing these specific aims will be applied to explain the functions of a variety of enzymes and proteins that derive their activity from PCET.